The present invention relates to a magnetic material sputtering target that is used for forming a magnetic thin film for a magnetic recording medium, in particular, a granular film for the magnetic recording medium of a hard disk employing a perpendicular magnetic recording system, and relates to a non-magnetic grain dispersion-type magnetic material sputtering target that can inhibit abnormal discharge due to an oxide causing occurrence of particles during sputtering, and a method of producing the target.
Though various systems are known as sputtering apparatuses, in formation of magnetic recording films, magnetron sputtering apparatuses equipped with DC power sources are widely used because of the high productivity thereof. In magnetron sputtering, a high density of plasma can be concentrated near the front surface of a target by disposing a magnet on the back face of the target and leaking a magnetic flux to the front surface of the target, resulting in an increase in the deposition rate.
Meanwhile, in the development of magnetic materials in the field of magnetic recording represented by hard disk drives, ferromagnetic metals, such as Co, Fe, or Ni-based materials, are used as materials for magnetic thin films that carry out recording. For example, in recording layers of hard disks employing a longitudinal magnetic recording system, Co—Cr based or Co—Cr—Pt based ferromagnetic alloys mainly containing Co have been used.
In addition, in recording layers of hard disks employing a perpendicular magnetic recording system, which has been applied to practical use recently, composite materials composed of Co—Cr—Pt based ferromagnetic alloys mainly containing Co and nonmagnetic inorganic materials are widely used.
Many of magnetic thin films of magnetic recording media such as hard disks are produced by sputtering a magnetic material sputtering target composed of the above-mentioned materials because of its high productivity.
As methods of producing these magnetic material sputtering targets, a melting method and a powder metallurgy method are proposed. The method used for producing a target is chosen depending on the characteristics required for the target, and the sputtering target composed of a ferromagnetic alloy and nonmagnetic inorganic grains, which is used for forming a recording layer of a hard disk of a perpendicular magnetic recording system, is usually produced by the powder metallurgy method. This is because since the inorganic grains are required to be uniformly dispersed in the alloy base material, it is difficult to produce the target by the melting method.
For example, proposed is a method of preparing a sputtering target for a magnetic recording medium by mechanically alloying an alloy powder having an alloy phase produced by rapid solidification and a powder constituting a ceramic phase to uniformly disperse the powder constituting a ceramic phase in the alloy powder, and molding the resulting powder by hot pressing (Patent Document 1).
The target structure in this case appears such that the base links in a soft roe-like (cod roe-like) manner and SiO2 (ceramics) surrounds the base (FIG. 2 of Patent Document 1) or is dispersed in the form of strings (FIG. 3 of Patent Document 1). Though other drawings are unclear, they look like to show similar structures. Unfortunately, such a structure has problems described below and is not a preferred sputtering target for a magnetic recording medium. Note that the spherical substance shown in FIG. 4 of Patent Document 1 is not a structure constituting the target but a powder.
Even if an alloy powder produced by rapid solidification is not used, a magnetic material sputtering target also can be produced by weighing commercially available raw material powders as the components constituting a target so as to give a desired composition, mixing the powders by a known process with, for example, a ball mill, and molding and sintering the powder mixture by hot pressing.
For example, proposed is a method of preparing a sputtering target for a magnetic recording medium by mixing a Co spherical powder, and a powder mixture prepared by mixing a Co powder, a Cr powder, a TiO2 powder and a SiO2 powder using a planetary motion mixer, and molding the resulting powder mixture by hot pressing (Patent Document 2).
The target structure in this case appears such that spherical phases (B) are present in a metal base phase (A) containing inorganic grains uniformly dispersed therein (FIG. 1 of Patent Document 2).
Though such a structure is advantageous for improving the leakage magnetic flux, it is not a suitable sputtering target for a magnetic recording medium from the viewpoint of inhibiting occurrence of particles during sputtering.
Furthermore, proposed is a method of preparing a sputtering target for forming a thin film of magnetic recording medium by mixing a Co—Cr binary alloy powder, a Pt powder, and a SiO2 powder and hot-pressing the resulting powder mixture (Patent Document 3).
It is described that the target structure in this case has a Pt phase, a SiO2 phase, and a Co—Cr binary alloy phase and that a dispersion layer is observed in the periphery of the Co—Cr binary alloy layer (not shown in drawing). Such a structure is also not preferred for a sputtering target for a magnetic recording medium.
In addition to the above, there are some proposals for developing magnetic materials. For example, Patent Document 4 proposes a perpendicular magnetic recording medium including SiC and SiOx (x: 1 or 2). Patent Document 5 describes a magnetic material target containing Co, Pt, a first metal oxide, a second metal oxide, and a third metal oxide.
Patent Document 6 proposes a sputtering target composed of a Co and Pt matrix phase and a metal oxide phase having an average grain diameter of 0.05 μm or more and less than 7.0 μm and proposes to increase film-deposition efficiency by inhibiting the growth of crystal grains to obtain a target with a low magnetic permeability and a high density.
Patent Document 7 describes a non-magnetic grain dispersion-type ferromagnetic material sputtering target, in which the ferromagnetic material is composed of Co and Fe as main components and a material selected from oxides, nitrides, carbides, and silicides and the non-magnetic material has a specific shape (smaller than a virtual circle of a radius of 2 μm).
Patent Document 8 describes a non-magnetic grain dispersion-type ferromagnetic material sputtering target, in which non-magnetic grains of an oxide smaller than a virtual circle of a radius of 1 μm are dispersed in a ferromagnetic material of a Co—Cr alloy and the grain diameter thereof is minutely regulated. Patent Document 9 describes a magnetic film having a granular structure.
As described above, in non-magnetic grain dispersion-type ferromagnetic material sputtering targets, such as Co—Cr—Pt-oxide, use of SiO2, Cr2O3, or TiO2 as an oxide has been proposed. In addition, specification of the shape of an oxide has been proposed. However, these oxides are insulative materials and thereby cause abnormal discharge, which causes a problem of generating particles during sputtering.
The floating amount of a magnetic head is reduced year after year accompanied by an increase in the recording density of HDD. Accordingly, the acceptable size and number of particles on a magnetic recording medium have become increasingly restricted. It is known that many of the particles generated during the formation of a granular film are an oxide caused from a target. As a method for inhibiting such occurrence of particles, fine dispersion of an oxide in the target into an alloy matrix is believed to be effective.
In addition to Patent Documents 6 to 8, Patent Documents 10 to 15 also propose finer grains of metal oxides. That is, Patent Document 10 describes grains formed by a metal oxide phase having an average grain diameter of 0.05 μm or more and less than 7.0 μm; Patent Document 11 describes a ceramic phase having a long-axis grain diameter of 10 μm or less; Patent Document 12 describes an oxygen-containing material or an oxide phase of 50 μm or less; Patent Document 13 describes grains formed by an oxide phase having an average grain diameter of 3 μm or less; Patent Document 14 describes silica grains and titanium grains satisfying the requirement: 2≦Dp/Dn in a cross-section perpendicular to the main surface of a sputtering target, wherein Dn denotes the grain diameter in a direction perpendicular to the main surface of a sputtering target, and Dp denotes the grain diameter in a direction parallel to the main surface; and Patent Document 15 describes restriction of the number of chromium oxide aggregates to 500 grains/mm2. Unfortunately, these conditions are still insufficient, and further improvement has been required yet.                Patent Document 1: JP H10-088333 A        Patent Document 2: Japanese Patent Application No. 2010-011326        Patent Document 3: JP 2009-001860 A        Patent Document 4: JP 2006-127621 A        Patent Document 5: JP 2007-004957 A        Patent Document 6: JP 2009-102707 A        Patent Document 7: Domestic Re-publication of International Publication No. WO2007/080781        Patent Document 8: International Publication No. WO2009/119812 A1        Patent Document 9: JP 2001-076329 A        Patent Document 10: International Publication No. WO2009-054369        Patent Document 11: JP 2006-045587 A        Patent Document 12: JP 2008-169464 A        Patent Document 13: JP 2009-215617 A        Patent Document 14: JP 2011-222086 A        Patent Document 15: JP 2008-240011 A        